Ultimately, employment advice from an assessment authority is just that: advisory.
In terms of teaching applicants, you can submit your employment evidence directly to immigration and make a claim for the points, without obtaining points test advice from the skills assessment authority (although you still need a positive skills assessment obviously).
However, it should be noted that an immigration case officer reserves the right to ask you to go back and get employment advice if they feel they would like the additional advisory.
For this reason, most people try to pre-empt this possibility from occurring by getting employment advice before applying for the visa. it comes down to individual choice.
I always advise people to go for it at the same time as their skills assessment, as then they are covered.
However, in situations where you are adding subsequent work experience AFTER a skills assessment e.g. you worked for an additional year in Australia, then there should be no need to get additional employment advice for this. You just 'add it on'. Providing you've factored this into the EOI and you have employment evidence then the case officer should accept it.
Regards,
Richard